This invention generally concerns the field of of polyurethane compositions. More specifically, it concerns chain extenders useful in polyurethane preparations, particularly chain extenders useful in polyurethanes prepared by reaction injection molding (RIM).
The term "polyurethane" is used generically herein to denote polymers produced by reaction of at least one polyisocyanate starting component and at least one polyfunctional active hydrogen containing starting component. Production of polyurethanes often involves more than one active hydrogen compound. When two or more active hydrogen components are used, at least one is generally a relatively high equivalent weight active hydrogen containing compound. Generally speaking, when incorporated into polyurethanes, relatively high equivalent weight active hydrogen compounds produce segments of polymer called soft segments, which segments have relatively low glass transition temperatures. Relatively low glass transition temperatures are those generally below the temperatures of intended use of a polyurethane.
Relatively low equivalent weight active hydrogen compounds, refered to herein as chain extenders, are often used in addition to the relatively high equivalent weight active hydrogen compounds in a polyurethane-forming composition. Chain extenders in polyurethanes generally produce high melting segments called hard segments, which are believed to result from an increased intermolecular association or bonding within the polymer. Intermolecular association or bonding can be by covalent or hydrogen bonds. An amount and type of chain extender is generally chosen to achieve preselected processing characteristics, preselected hardness and other preselected physical properties in a final polyurethane.
In reaction injection molding (RIM), chain extenders are chosen to impart desired physical properties to resulting molded objects and also to achieve a preselected reaction time, which is generally a reaction time sufficiently long to allow complete filling of a mold before gelling of the polymer, yet sufficiently fast to achieve short demold times. Demold times are typically 30 seconds to 2 minutes with a shot time, during which the components are injected into the machine, of about 5 seconds.
Reaction injection molding is a process for producing and molding polyurethanes, which process has proven especially useful for making large molded objects having resilience such as furniture items: light weight building materials; shoe soles and heels: industrial service parts such as rollers, gears, bearing pads, pump housings auto body parts such as bumpers, fascia, hoods, doors, and fenders and the like.
The RIM technique involves filling a mold with a stream of reactive, liquid starting components, which are rapidly injected. The starting components mix by impingement as they are injected into a mixing head, from which they flow rapidly into the mold. On mixing, the components quickly begin to react to form polyurethanes, including those having urea bonds. The mixture should remain fluid for a period of time sufficient to fill the mold, which is often of large volume and complex design. The period of time required to fill the mold is the filling time. The period of time between initial mixing of starting components and the first visible reaction in the form of creaminess of the reaction mixture is called cream time. The period of time between initial mixing and formation of sufficient gel to solidify the polyurethane is called gel time. Shortly after gelling, the polyurethane cures to form a molded object having sufficient dimensional stability that it can be removed from the mold, leaving an empty mold for another injection of starting components. The time from injection until such an object is formed is called the demold time. The demold time is but one component of the cycle time, which is a measure of the time from one injection until a RIM machine is ready for the next injection of starting components. Cycle time includes time necessary for mechanical operations such as opening and closing a mold, time required for mold preparations such as removing flash left after demolding, applying external mold release agents, and the like.
Starting components enter the mold, gel, cure and are removed very rapidly. Typically, the mixed stream of components enters the mold at a rate of from about 20 to about 1200 pounds per minute (from about 9 to about 545 kilograms per minute). Typical gel times range from less than about one second to about fifteen seconds. Cycle times are typically from about three to about five minutes and with high speed equipment are often less than about two minutes. Every stage of the process is preferably optimized to achieve cycle times which are as short as possible.
When RIM is used in a one shot process of producing polyurethanes, the polyisocyanate starting component is supplied to the mixing head in one stream and the active hydrogen starting components are supplied in another stream. In a two shot process, a prepolymer is supplied in one stream while remaining active hydrogen components are supplied in the other. Prepolymers having isocyanate groups are included in the term "polyisocyanate component." The stream of polyisocyanate component(s) is referred to as the "A side", or "A component" while the stream containing the active hydrogen component(s) is referred to as the "B side", or "B component." Chain extenders, catalysts, and additives are typically blended and introduced in the B side, but in certain cases catalysts and some additives can be mixed into the "A side". In some instances a third or fourth stream is required to meter an active ingredient which is incompatible with, prematurely reacts with or otherwise cause difficulty with one or more of the other components of the polyurethane. The term "reaction mixture" as used herein refers to an admixture of at least one polyisocyanate component and at least one hydrogen component which will form a polyurethane. Additives are optionally included in a reaction mixture.
It is important in RIM that starting components be mixed in desired proportions and that they be mixed intimately. It is also important that inclusion of air in the molded product not be in visible bubbles or pockets. The mold must be filled completely. These and other requirements are met by processes known to those skilled in the art. Such processes include processes disclosed in U.S. Pat. Nos. 3,709,640; 3,857,550; 4,218,543; 4,298,701; 4,314,962; and U.S. Pat. No. 4,582,887, which are incorporated herein by reference. Additional description of RIM processes is found in Prepelka and Wharton, "Reaction Injection Molding in the Automotive Industry," Journal of Cellular Plastics, vol. 2, no. 2, pp. 87-98 (1975) and Knipp, "Plastics for Automobile Safety Bumpers," Journal of Cellular Plastics, pp. 76-84 No. 2 (1973).
Chain extenders have important effects on processes for producing polyurethanes, particularly on RIM processes. If a chain extender reacts too rapidly, the polyurethane reaction mixture may gel before a mold is completely filled or before a foam has reached its greatest volume. If the chain extender is too slow, each cycle time will be increased by the time necessary for the reaction mixture to react sufficiently to form a demoldable polyurethane. Those skilled in the art know that the reactivity of the chain extender must be appropriate for use with the other components of a polyurethane formulation to achieve desired reaction times. At the same time, the chain extender must act together with the other components of the reaction mixture to yield a polyurethane having the desired physical properties.
A number of chain extenders have been suggested for use in polyurethane formulations. Aromatic diamines having at least one linear alkyl substituent of one to three carbon atoms in a position ortho to an amine group, such as those disclosed in U.S. Pat. No. 3,428,610 and U.S. Pat. No. 4,218,543 are commonly used as chain extenders. Alkyl substituted methylene dianilines such as those disclosed in U. S. Pat. Nos. 4,294,934 and 4,578,446 may be used alone or in combination with the substituted aromatic diamines. Variations of substituted aromatic diamines such as the aromatic diamines having an electron withdrawing group disclosed in U.S. Pat. No. 4,523,004; the vicinal toluenediamine disclosed in U.S. Pat. No. 4,529,746; the dialkyl aromatic diamines disclosed in U.S. Pat. No. 4,526,905 and oxyethylated toluenediamines disclosed in U.S. Pat. No. 4,596,685 have been proposed for use as chain extenders in RIM produced polyurethanes. Combinations of chain extenders such as the combinations disclosed in U.S. Pat. No. 4,269,945 which include at least one primary amine have also been proposed for use in polyurethanes formed in RIM processes.
Combinations of reactivity and physical properties such as hardness, modulus and load deflection and the like imparted to resulting polyurethanes by known chain extenders are limited. It would be desirable, for instance, in situations involving large complex molds, to achieve the hardness attainable with some known substituted aromatic diamines yet to have somewhat slower reaction times to allow complete filling of molds. It is also desirable, in many applications, to produce unfilled polyurethanes having physical properties such as thermal stability and modulus generally associated with the use of fillers in polyurethanes produced from formulations including known chain extenders.